Reflective radar target



June 12, 1962 c. ROCKWOOD REFLECTIVE RADAR TARGET Filed May 31, 1956 Miq3,039,093 REFLECTIVE RADAR TARGET Charles H. Rockwood, Evanston, 11].,assignor, by mesne assignments, to Cook Electric Company, Chicago, 11].,a corporation of Delaware Filed May 31, 1956, Ser. No. 588,347 3 Claims.(Cl. 343-18) This invention relates to an improved electromagneticreflector and more particularly to an improved radar reflector having alarge effective cross section and substantially omnidirectionalcharacteristics.

A theoretically perfect omnidirectional radar reflector, that is areflector which will reflect incident waves uniformly in all directionsis a sphere. However, the convex reflective surface which a spherepresents to incident Waves substantially reduces the percentage oftransmit-ted energy which is reflected back to a receiver. The use ofvarious shaped directional reflectors has been investigated extensivelyand the theoretical optimum unidirectional response of a reflector isproduced by a perfectly conductive planar member in which the incidentradar energy is transmitted toward the plane normal thereto. However,such a reflector is highly directional and even a slight departure fromthe normal axis produces a substantial reduction in the usuablereflected energy.

It is therefore one particular object of this invention to provide animproved reflector for electromagnetic waves having a large effectiveelectromagnetic cross section.

-It is another object of this invention to provide an improvedelectromagnetic reflector having a large effective cross section over asubstantial range of directions.

It is a further object of this invention to provide an omnidirectionalreflector for electromagnetic waves which has a large substantiallyuniform electromagnetic cross section for incident energy from anyarbitrary direction.

It is a still further object of this invention to provide an improvedomnidirectional reflector for electromagnetic waves which may beconstructed of relatively simple and inexpensive parts with a minimumcost of fabrication.

Further and additional objects of this invention will become manifestfrom a consideration of this description, the accompanying drawings andthe appended claims.

In one form of this invention a solid reflector is provided comprising aplurality of triangular corner reflectors disposed in edge to edgerelationship whereby the outer edges thereof form a portion of anicosahedron. For the purposes of this description, the triangular cornerreflectors may be termed trihedral angles or trihedrons. Moreparticularly, fifteen right trihedrons are assembled in edge to edgerelationship with their apexes directed inwardly toward a common centerand the planes of their outer edges defining an icosahedron with fiveadjacent surfaces removed.

For a more complete understanding of this invention reference will nowbe made to the accompanying drawing wherein:

FIG. 1 is a perspective view of one embodiment of this invention;

FIG. 2 is a reduced bottom plan view of the embodiment of FIG. 1;

FIG. 3 is a top plan view of the embodiment of FIG. 1, this figure alsorepresenting the bottom view of a modified embodiment;

FIG. 4 is a bottom plan view of the embodiment of FIG. 1 with the bottomplate removed to indicate the internal construction of the reflector;and

FIG. 5 is a perspective view of one trihedron forming a part of theembodiment of FIG. 1.

Referring now to the drawings and more particularly 3,39,ii93 PatentedJune 12, 1962 to FIG. 1, a radar reflector is illustrated mounted on avertical post .12. The radar reflector comprises a total of forty-fivetriangular surfaces 14- disposed in a unique manner to produce optimumreflection of incident radar waves from all directions above the ground.

The faces 14 are assembled together in sets of three,

as clearly illustrated in FIG. 5, to form trihedral corner reflectorscomprising three faces 14a, 14b, and 14c, secured together alongadjacent edges and having a common apex 16. In the preferred embodimentof this invention the corner reflector 18 is formed very precisely asthe corner of a cube. That is, the angle formed by any two faces '14 ofthe trihedron 18 will be 90 when measured normal to the common edgethereof. Thus, the angle formed by faces 14a and 14b when measurednormal to the common edge 26a is 90". Similarly, the

' angle formed between faces 14b and 140 measured norsides. In a regularicosahedron, the sides are triangular and are joined in edge to edgerelationship insuch a manner that at every apex of the figure five sidesmeet. In the instant embodiment each of the planar portions forming aside of the icosahedron are defined by the edges 24 and the area definedby these edges is open with the three triangular faces 14 defining ahollow trihedral angle therein.

In the particular embodiment described, the bottom fifteen triangularfaces which would define the bottom five icosahedron sides have beenremoved and a flat plate 22 substituted therefor whereby the reflectoris especially adapted for ground mounting. In the particular embodimentdescribed the vertical post 12 is secured to the bottom plate 22 and thepost will normally maintain the reflector above the ground at apredetermined height for most efficient signal reflection. As allincident signals will be arriving at the reflector parallel to or abovethe ground plane, the removal of the lower sides of the icosahedron willin no way impair the efliciency of the reflector.

It is contemplated that the reflector of this invention may be utilizedin many ways. For example, a reflector may be used as a drop from flyingaircraft, missiles and the like for testing, military countermeasures,as decoys and the like. In such operations it will be desirable toutilize the entire symmetrical figure comprising a total of sixtytriangular faces formed into twenty corner reflectors or trihedrons as acomplete balanced solid in the nature of an icosahedron.

Referring now to FIG. 4, a bottom view with the bottom plate removedillustrates the internal construction of the reflector 10. Therein itcan be seen that the lower five trihedrons 18a, 18b, 18c, 18d and lfleextend into the reflector with the apexes thereof directed to the centerof the body but disposed from the center a substantial distance.Intermediate the lower trihedrons 18a-e are trihedrons 18g-k which canbe clearly seen in FIGS. 1, 3 and 4. The combination of the lowertrihedrons 18a-e and the intermediate trihedrons 18g-k form a closedhollow body open at the top and bottom. A top assembly ting edgesdescribed and illustrated may be secured together in any mannerdepending upon the nature of the material employed. The only requirementin choosing materials for use in this invention is that the outersurfaces of the faces have good reflective properties forelectromagnetic waves.

In the event that a complete solid is desired, a second star-likeassembly identical to the upper assembly comprising stnlctures 18m-q maybe constructed and secured to the lower edges 24d-h of the illustratedembodiment. The bottom view of the reflector will then be as shown inFIG. 3, rather than as shown in FIG. 2. As is clear from the showing ofFIG. 4 the apexes of all the trihedrons 18a-q are directed toward thecenter of the solid but are disposed a substantial distance therefrom.The precise spacing of the apexes from the center will, of course, bedetermined by the nature of each trihedron, the shape of the facesthereof, and the angle therebetween. It has been found that optimumresponse is accomplished through the utilization of the particular typetrihedron described above in which the various faces are disposed innormal relationship. In such a construction the assembly illustrated inFIG. 4 necessarily results.

The advantages of a reflector constructed in accordance with thisinvention may be roughly determined for certain theoreticalcalculations. While these calculations are theoretically accurate indetermining the approximate response of various reflectors, it has beenfound in practice that the actual radiations or reflections varysubstantially from the calculated values. However, the reflectordescribed hereinabove has proven more uniform throughout the entirerange of azimuth and elevation than any reflector heretofore known ofcomparable size and complexity.

In a typical assumed situation, it has been theoretically determinedthat the same power ratio (that is the same ratio between incident andreflected power) will be obtained from targets having the followingconfigurations and dimensions: A sphere having a 270 ft. diameter forincident energy having a 1 cm. wavelength, a sphere of 90 ft. for 3 cm.wavelength energy, a sphere of 27 ft. for cm. wavelength energy, and areflector of the type described having a corner length of 2 ft. for allwavelength small with respect to the corner length.

This was determined by estimating the effective radar cross section of asphere from the equation:

The effective radar cross section of a single corner reflector forincident energy along the corner axis is estimated from the equation:

where a is the corner length and A is the wavelength.

These equations may be substituted in the general radar equation forcalculating power ratio in a closed reflection system:

P, (44r)3R wherein P is the power received at the receiver; P, is thetransmitted power; G is the antenna gain; R is the radar range; and F isthe pattern propagation factor. For the purposes of this study, it isassumed that the pattern propagation factor F is a constant, namely,unity, and that the radar range, and antenna gain are fixed. Thus thepower ratio is a function of effective radar cross section andwavelength only.

As will be clear, substituting Equation 2 in Equation 3 will eliminatethe factor and thus power ratio from a corner reflector is independentof wavelength.

Theoretical calculations and tests indicate that an assembly oftriangular corner reflectors as shown and described above produceaverage minimum reflection characteristics of the order of 70% of thereflection along the axis of one corner reflector in all directions andthat the reflection is sufliciently uniform to maintain reliable radarindications for all angles of incidence. Thus by multiplying thecombined equation of Equations 2 and 3 by the factor .7, the minimumpower ratio for a reflector as taught by this invention is determined.

Without further elaboration, the foregoing will so fully explain thecharacter of my invention that others may by applying current knowledge,readily adapt the same for use under varying conditions of service,while retaining certain features which may properly be said toconstitute the essential items of novelty involved, which items areintended to be defined and secured to me by the following claims.

I claim:

1. A reflective radar target comprising a closed polyhedron having sixtyplane surfaces in the shape of right triangles, said surfaces being madeof a material which is reflective to electromagnetic radar radiation,said surfaces being joined edge-to-edge and being arranged in twentyopen trihedrons having open sides facing outwardly and apexes pointinginwardly toward the center of said polyhedron, each of said trihedronsbeing composed of three of said surfaces joined at right angles to oneanother in a configuration corresponding to an internal corner segmentof a cube.

2. A reflective radar target comprising at least a seg-- ment of apolyhedron having sixty plane surfaces in the shape of right triangles,said surfaces being made of a material which is reflective toelectromagnetic radar radiation, said surfaces being joined edge-to-edgeand being arranged in open trihedrons having open sides facing outwardlyand apexes pointing inwardly toward the center of said polyhedron, eachof said trihedrons being composed of three of said surfaces joined atright angles to one another in a configuration corresponding to aninternal corner segment of a cube.

3. A reflective radar target, comprising a flat base plate in the shapeof a pentagon and surmounted by a forty-five sided segment of asixty-sided polyhedron having plane surfaces in the form of righttriangles, said surfaces being made of a material which is reflective toelectromagnetic radar radiation, said surfaces being joined edge-to-edgeand being arranged in fifteen open trihedrons having open sides facingoutwardly and apexes pointing toward the center of said polyhedron, fiveof said surfaces having edges joined to said pentagonal base plate, eachof said trihedrons being composed of three of said surfaces joined atright angles to one another in a configuration corresponding to aninternal corner segment of a cube.

References Cited in the file of this patent UNITED STATES PATENTS2,576,255 Hudspeth et a1 Nov. 27, 1951 2,746,035 Norwood May 15, 19562,763,000 Graham Sept. 11, 1956 FOREIGN PATENTS 696,834 Great BritainSept. 9, 1953 OTHER REFERENCES Cundy et al.: Mathematical Models byCundy and Rollett, QA11C8, published at Oxford at the Clarendon Press,Oxford University Press, Amen House, London, first edition 1951,reprinted lithographically in Great Britaim at the University Press,Oxford, from corrected sheets of the first edition 1954, 1956, 1957, seepages 13, 82, 86 and Plate 1.

